In many wireless communications systems, Channel-State Information, CSI, feedback is crucial for obtaining good performance between a transmitting entity and a receiving entity. For example, the transmitting entity transmits references signals that provide the receiving entity with a basis for estimating channel state. Reported CSI feedback from the receiving entity typically includes a Channel-Quality Indicator, CQI, a Rank Indicator, RI, and a Pre-coding Matrix Indicator, PMI. The CQI value servers as a quantized representation of measured signal quality, the RI value indicates the number of transmission layers that can be supported, and the PMI value indicates a preferred precoder—i.e., a preferred set of antenna weights to be used for performing a multi-antenna transmission from or to the reporting entity. In general, the entity reporting CSI and the entity receiving the CSI report have knowledge of a defined codebook that contains some number of precoders, wherein the PMI “points” to a preferred one of the precoders within the codebook.
The Third Generation Partnership Project, 3GPP, Long Term Evolution, LTE, system supports CSI-reporting schemes that rely on the reference symbols being transmitted periodically. The LTE radio structure is based on recurring frames of a defined duration, with each frame subdivided into a regular number of subframes. In this context, cell-specific reference symbols, CRS, are sent every subframe, for example, while user-specific CSI Reference Symbols, CSI-RS, may be sent with a larger periodicity. User Equipments, UEs, using Transmission Mode 10, TM10, rely solely on CSI-RS resources, while other UEs typically use the CRS at least for interference measurements.
UEs operating in an LTE system transmit CSI reports either on the Physical Uplink Control Channel, PUCCH, or the Physical Uplink Shared Channel, PUSCH. CSI reporting on the PUSCH generally involves the transmission of CSI along with whatever data is being transmitted on the PUSCH. TM10 UEs can be configured to report CSI for multiple CSI-processes, which each may have different CSI-measurement resources. A CSI-measurement resource, CSI-MR, consists of a CSI-RS resource and a CSI Interference Measurement, CSI-IM, resource. Both the CSI-RS and the CSI-IM resources are divided into sets of resources, where each set is identified by CSI-RS configuration index. Each CSI-RS configuration index contains resources in every Physical Resource Block, PRB, in the involved frequency band. A subframe configuration specifies a subframe periodicity and a subframe offset that specify for the UE at which time instances the respective measurement resources are available.
As the number of antenna elements used at access nodes in the radio network increases, the size of the precoder codebooks used for precoding from these larger sets of antenna elements also increases. In the early releases of LTE, the number of different precoders was rather limited. For example, for two antenna ports, four rank-1 and two rank-2 precoders were specified. For four antenna ports, sixteen different precoders were specified. The number of bits in the CSI report to indicate the desired precoder was limited to two and four bits for two and four antenna ports, respectively. However, Release 11, R-11, of the 3GPP specifications extended precoder support up to eight antenna ports, resulting in a significant expansion in the size of the precoder codebook. For example, for rank one and two, eight bits is required to indicate the desired precoder. Release 13, R-13, extended precoding support for up to sixteen antenna ports and providing for up to eight transmission layers, i.e., Rank 8 transmissions, with over-sampling of the precoder codebook. These changes again increased the number of bits needed to indicate a desired precoder from the precoder codebook. For example, using sixteen antenna ports and with configuration parameters (N1, N2) . . . , nine bits is required to indicate a desired precoder from the defined codebook, if the rank is at most two.
As the number of antenna ports increases, the number of feedback bits required in the CSI report increases. While the increased overhead may not be significant when reporting CSI over PUSCH, PUCCH is a scarce resource shared among all UEs in a cell, and significantly increasing CSI reporting overhead on the PUCCH is problematic.
For the Fifth Generation, 5G, systems now under development—e.g., systems using the “New Radio” or NR interface now being standardized—the number of transmit antennas on the network side are expected to increase dramatically, as compared to current systems. For example, a radio access node may be equipped with several hundred antennas (or antenna elements), allowing sophisticated beamforming. It is recognized herein that existing approaches to precoding control, including existing approaches to evaluating and reporting preferred precoders, do not scale well as the number of antenna ports increases.
As one example, consider that a UE or other wireless devices needs to allocate significant computational resources when evaluating large codebooks to identify preferred precoders. It may be difficult, for example, for a wireless device to evaluate large sets of precoders within the time constraints associated with ongoing communications. Even allowing for continued improvements in the processing capabilities of wireless devices, however, the power expended on large sets of computations will negatively affect battery life of such devices.
To see the complexity associated with evaluating a set of precoders, consider the well-known Maximum Mean Square Error, MMSE, receiver, wherein a precoder P, from a set of precoders S, is determined such that the Signal-to-Noise-and-Interference-Ratio, SINR, representing a quality estimate q(P) is maximized. Hence, the problem becomes
  P  =      arg    ⁢                  ⁢                  max                  P          ∈          S                    ⁢                        q          ⁡                      (            P            )                          .            
To determine the SINR for layer l for a channel matrix H and interference and noise covariance matrix Q when using the precoder P, the following computations may be carried out at the wireless device:
      R    =          (                                    HPP            *                    ⁢                      H            *                          +        Q            )            W    =                  P        *            ⁢              H        *            ⁢              R                  -          1                                Q      x        =          I      -      WHP      +              WQW        *                                SINR        l            ⁡              (        P        )              =                            (                                    [              WHP              ]                                      l              ,              l                                )                2                              [                      Q            x                    ]                          l          ,          l                                q      ⁡              (        P        )              =                  ∑        l            ⁢                        SINR          l                ⁡                  (          P          )                    
The preceding computations involve complex matrix multiplications and inverses and some computations involve P and require that q(P) is evaluated per P, i.e., per precoder being evaluated. This fact means that when evaluating which precoder is preferred, a large number of overall computations is required when the overall set of precoders is large, i.e., when the codebook is large.